The invention concerns the field of microelectronics packaging technology. Various bonding methods and mediums may be used to attach the IC die. Since the development of the flip chip mounting process using solder by IBM (C4 process) in the early 1960's, the face down bonding technology has become popular and manifold and original interconnection methods have been reported. At the same time the interest in flip chip bonding is being driven by demands for substrates accepted in the consumer field. Therefore the substrates which are to be bonded by flip chip have been changing from inorganic to organic, such as polyimide foil and printed circuit boards (often FR-4). By using solder alloys it is possible to attain low resistance as well as a good thermal contact between the chip and the packaging substrate, including FR-4 and polyimide flex with the use of encapsulants. The result is an improvement in the thermal life cycle of flip chips mounted on high CTE packaging substrates.
The interest in flip chip assemblies using adhesives for high density, fine pitch and high performance interconnections has increased rapidly. In this context a new flip chip technology is suggested using non-conductive adhesives in place of the known stiffenable resin (cf. U.S. Pat. No. 4,749,120). The concept is to simultaneously attach and electrically interconnect bare chips with gold stud bumps to many types of packaging substrates, the bumps having a certain shape to ease the piercing through the sheet or sheet-foil of adhesive. By this interconnection method, the chip is bonded face down and is electrically connected via compressed and deformed gold ball bumps with the organic substrate. The chip is fixed by the non-conductive adhesive film which fills the entire gap between the die (the bare chip) and packaging substrate. The new method obtains greater compliance or flexibility, especially advantageous for organic substrates.
Compared to U.S. Pat. No. 4,749,120 (Hatada, Matsushita) the bonding by heat was extraordinarily accelerated although physical bonding parameters were improved and flexibility (compliance) of the bonded microelectronic package was obtained.
The adhesive is best to be applied in sheet foil form.
An exemplary non-conductive adhesive film was studied with an emphasis on the properties of COF (chip on flex) and COB (chip on board) interconnections. Electrical and mechanical performance of the adhesive bonds were studied by evaluating initial contact resistance and mechanical adhesion as a function of temperature and humidity.
Both mechanical and electrical properties were measured before and after the environmental tests and compared to soldered contacts. For flip chip interconnects with a pad size of 100 .mu.m.sup.2 on the chip site and a contact area of approx. 60 .mu.m round, the resistance is less than 8 m.OMEGA.. This low contact resistance can be attributed to the special process and materials applied according to the invention. Moreover, the bonding pressure, bonding temperature and time in comparison with the contact resistance were examined.
The results of the inventive concept indicate that the control of these process parameters will yield good bonding quality. Non-conductive adhesive flip chip technique offers several important advantages over solder filling materials, however, introduce other new problems. The major disadvantages are that they do not allow rework, they have manufacturability problems such as long cure time and they need high solder bumps. The use of adhesives instead of soldering in flip chip bonding on organic substrates avoids the problems with solder. Also there are potential cost benefits in the reduction of processing steps if adhesive flip chip bonding is used.
The invention is based on stud bumps and non-conductive adhesives. This technique allows quality attachment of IC's on low cost organic substrates and offers numerous advantages in the assembly of electronic circuits. These include low processing temperatures, fluxless bonding, high density interconnections, replaceability and high reliability, speeding up of the attachment process and allowing for elastic movement of the bonded microelectronic package.
The shape of the bumps is a base and a peak section being connected via a conical section; they may be of gold, to yield best results. To improve and ease the deformation during bonding, the protruding front section is softer than the base part of the bumps.
Examples and test results will demonstrate the invention in greater detail.